The invention relates to a channel accessing process for a local transmission network, configured as a bus system for message with packet classification without central control, in which each station able to transmit monitors the transmission channel and--depending on the state of the transmission channel--transmits its message and wherein, in the case of a collision of messages, an interruption of the message transmission and a collision resolution takes place on the communications channel in a resolution phase.
With such a process for network accessing, it is a basic principle that each station able to transmit can spontaneously access the transmission channel. During this process, collisions must be avoided if possible. Each station which has a message to send, tests the communications channel as to whether it is occupied or free. If it is free, then the message is transmitted. If, during this process, a collision occurs because another participant--also having checked the communications channel and having found it to be unoccupied--is transmitting at the same time, then this collision is detected and both stations repeat their initially futile attempt, at which time--due to the activation of random generators--a random time displacement can be introduced at both participating stations. For this reason and with great probability, subsequently only one message transmission [at a time] is sent to the communications channel. For this type of message transmission, a so-called transmission protocol is always essential. The most known one is the so-called CSMA/CD process (carrier sense multiple access with collision detection).
Each station of the message transmission network must contain a control unit for its control in accordance with the prescribed transmission protocol, by means of which it is assured that all stations operate in the same way. With the initially described channel access process, in case of a great load on the communications channel due to collisions which at that time are more frequent, the efficiency of the entire system can deteriorate, [a situation] manifesting itself by a prolonged average data transfer time. With the introduction of new data transmission services with large amounts of information, such as access to distributed memories or image transmission, the load of the message transmission network becomes greater. Typically, the basic load in a data network lies below 10%; however, with steadily increasing traffic, load peaks in the range of 30 to 40% occur more and more frequently. For this reason, there exists the danger of an overload, as well as unacceptable response times for the users.
Attempts have already been made to reduce the number of collisions which necessarily occur with a process of the initially named type by developing particularly advantageous algorithms for the collision resolution.
Also, in order to shorten the data transfer time during the data transmission in packet transmission networks, priorities were raised for discussion, wherein the packets in each packet switching device are sorted according to length and the transmission of short packets is preferred. For this purpose, only collision-free systems with central control are taken into consideration and no substantial improvements are achieved because the packet lengths scatter only relatively weakly.